1 Field of the Invention
The present invention concerns the wheel suspension for a vehicle, in particular for utility or agricultural vehicles, with a hydro-pneumatic spring system arranged between the vehicle body and the wheel support.
2. Description of the Related Art
Wheel suspensions for vehicles frequently include at least one hydraulic cylinder, a hydraulic reservoir arrangement and an intervening valve arrangement, e.g., as shown in DE 28 07 299.
DE 38 34 693 describes a tractor with a steerable rigid axle supported a transverse control arm. A vertical spring damper is provided between the front axle support and the transverse control arm, and includes a spring loaded hydraulic cylinder. Damping is provided by a throttling restriction arranged in the connection between the hydraulic cylinder and a hydraulic reservoir. Furthermore, a two-way valve also is arranged in the connection which selectively blocks the spring damper by blocking the supply or return flow to the hydraulic cylinder. This known spring arrangement is effective only in a limited pre-set load range, dependent upon the operating range of the hydraulic reservoir. For support loads outside this load range no spring loaded wheel suspension is provided.
Especially in utility and agricultural vehicles, the axle loading or the contact forces of the vehicle body upon a spring-loaded axle may vary within wide limits due to changes in the load. In tractors with a mid-range load capacity, the load applied by the body to the front axle can reach 4000 Newtons in plowing and 48,000 Newtons in front loader operations, so that the maximum load required is 12 times the minimum load. These varying loads must be absorbed by spring arrangements with appropriate spring forces and appropriate hydraulic damping forces. Use of a spring arrangement for the front axle is especially recommended in tractors with a high top speed, both to improve the driving safety of the tractor and to increase operator comfort.
Experience in vehicle technology has shown that the natural frequencies of the vehicle body should be kept nearly constant under all load conditions. As a result of this rule the spring rate and the damping rate must be made approximately proportional to the change in load.
It is known from truck practice to supply the required amount of air from an air compressor through an air reservoir to air springs in order to cover varying load conditions. In many vehicles, in particular in tractors, no compressed air system is available, so that here the truck solution would be too costly. Furthermore, air springs have a considerably greater space requirement than do hydro-reservoir systems due to their relatively low internal pressure, and the required space is frequently not available.